Several different solutions are known which can modify (in particular, increase) the perceived spatial width/stereo image of a stereo signal.
One family of approaches for stereo widening relies on a simple linear processing that can be done in the time domain. In particular, the stereo signal pair can be transformed to a mid (sum of both channels) and side (difference) signal. Then, the ratio of side to mid is increased, and the transformation is reverted to obtain a stereo pair. The effect is to increase the stereo width. These methods belong can mainly be classified as an “internal” stereo modification approach, although the stereo width can theoretically also be extended beyond the loudspeaker span. The computational complexity is very low, but there are several disadvantages of such methods. The sources are not only redistributed among the stereo stage, but also weighted, spectrally, differently. That is, the spectral content of the stereo signal is modified via the widening process. This can degrade the audio quality. For example, the level of reverberation (which is included in the side signal) can be increased, or the level of center-panned sources (such as voices) can be decreased. Examples of such approaches are found in EP 06 772 35B1 and U.S. Pat. No. 6,507,657B1.
Another approach for stereo widening is cross-talk cancellation (CTC), which can be classified as an “external” stereo modification. The goal of CTC is to increase the stereo width beyond the loudspeaker span angle or, in other words, virtually increase the loudspeaker span angle. To this end, such methods filter the stereo signals to attempt to cancel the path from the left loudspeaker to the right ear, and vice versa. However, such an approach cannot overcome limitations in the signals, e.g. when the signal does not use the full stereo stage. Further, CTC introduces coloring artifacts (i.e., spectral distortion) which deteriorate the listening experience. In addition, CTC works only for a relatively-small sweet spot, meaning that the desired effect can only be perceived in a small listening area. One example of CTC is given in U.S. Pat. No. 6,928,168B2.